1. Field of the Invention
The invention relates to rotary drill bits for use in drilling holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of circumferentially spaced blades on the bit body extending outwardly away from the central axis of rotation of the bit, and a plurality of cutting elements mounted along each blade.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preformed (PDC) cutters each formed, at least in part, from polycrystalline diamond. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
The cutters on the drill bit have cutting edges which, together, define an overall cutting profile which defines the surface shape of the bottom of the bore hole which the bit drills. Preferably the cutting profile is substantially continuous over the leading face of the bit so as to form a comparatively smooth bottom hole profile.
In some drill bits of the above kind, there are associated with at least some of the cutters further secondary cutters each of which is circumferentially spaced from an associated primary cutter but is disposed at substantially the same distance from the axis of the bit as the associated primary cutter, so as to "track" the primary cutter as the bit rotates. That is to say, the secondary cutter follows the groove cut in the formation by its associated primary cutter as the bit rotates. In such arrangements the secondary cutters may be so disposed that their cutting edges lie inwardly of the profile defined by the primary cutters so that each secondary cutter serves as a back-up to its associated primary cutter and only performs an effective cutting action on the formation should the primary cutter become damaged or worn so that it is no longer effective.